In today's deregulated energy markets, covering peak load demand and the rapid provision of reserves of power has become a lucrative source of income for power plant operators. A number of methods for increasing power, such as the injection of water or steam in the combustion chamber, cooling or intercooling of the inlet air or injection of water into the compressor, are known measures for covering peak loads. The prior art in this context is a simple “on/off function”, but with all known methods it is typical to expect an increase in the wear to parts that are critical to service life.
For example U.S. Pat. No. 6,012,279 describes a method for operating a gas turbine group in which either the inlet air into the compressor or the partially compressed air between compressor stages, or both locations, is cooled. The cooling is in this case effected by injection of water and is not controlled. The result of the injection is that the temperature of the air to be compressed further is reduced, leading to a reduction in the compressor work. Moreover, the injection of water leads to an increase in the overall mass flow. Both the effects lead to an increase in power.
U.S. Pat. No. 5,463,873 on the other hand describes a method for operating a gas turbine group in which the air is cooled with the aid of what is described as “fogging” before it enters the compressor. In this case, the intake air is cooled by evaporation of water, producing a humidity level close to the saturation limit, before it enters the gas turbine. The maximum steam saturation of the inlet air is used as a control variable for controlling the fogging. A similar method is also described, for example, by U.S. Pat. No. 6,250,064.
Furthermore, U.S. Pat. No. 5,622,044 describes a method for operating a gas turbine, in which a preliminary compressor is connected upstream of the gas turbine. This preliminary compressor is driven by a separate electric motor, and the inlet air of the preliminary compressor is cooled by means of a water spray, as optionally is the pre-compressed air emerging from the preliminary compressor. The objective in this context is to allow the gas turbine group, in terms of its power, to operate as independently as possible of the ambient temperature.
A further document to be mentioned in this context is U.S. Pat. No. 6,216,443 which describes a gas turbine in which the power is increased by the injection of water droplets between intake muffler and compressor inlet, with the droplet size being in the range from 1 to 50 μm. In this context, the procedure is that to increase the available power of the gas turbine installation by a defined percentage, a defined quantity of water to be injected is calculated. This quantity is then fed to the inlet air by opening a control valve for supplying water accordingly. During this time, the supply of fuel is kept at a constant level. Only thereafter is the supply of fuel into the combustion chamber increased. One problem with this procedure is the fact that the injection of water upstream of the compressor leads to a change in the combustion temperature in the combustion chamber, and accordingly stability problems may arise as a result of the procedure described in this document. To summarize, with a procedure in accordance with U.S. Pat. No. 6,216,443 the setting of the water injection is operated in an open-loop control circuit. In particular, it should be noted that the setting of the water quantity or the cooling power and the setting of the fuel quantity take place sequentially, i.e. one after the other in terms of time. Furthermore, it must be noted that according to the procedure disclosed in U.S. Pat. No. 6,216,443, it is impossible to ensure that the machine will actually operate at its nominal full load and therefore generally at its optimum efficiency. For example, if the precalculated water quantity is too large, the gas turbine, on reaching the set power, will either be operated below its maximum firing temperature, or, if applicable, will not be operated with a fully open inlet guide vane row. Both of these problems are harmful to the gas turbine efficiency.